1. Field of the Invention
The present invention relates to a pulse launcher for single wire transmission lines adapted to pulsed electromagnetic sensors, and more particularly to fluid and material level sensors employing Guided Wave Radar (GWR) reflectometers. These sensors can be used for determining or controlling the fill-level of a tank, vat, irrigation ditch, silo, pile, or conveyor.
2. Description of Related Art
Propagation of electromagnetic waves along a single wire transmission line, also known as an electromagnetic guide, or guide wire, was first theorized by A. Sommerfeld in 1899. Sommerfeld's transmission line relies on a resistively lossy conductor to slow the wave slightly. Slower propagation, on the order of 99% of the speed of light, is required to retain the propagating fields proximal to the guide wire. In 1954, G. Goubau disclosed in U.S. Pat. No. 2,685,068, “Surface Wave Transmission Line,” another method to slow propagation without requiring a lossy conductor—a line coated with a dielectric material such as plastic. Goubau showed, contrary to conventional notions that guide wires are very leaky, that his coated guide wire exhibited the lowest loss of any transmission line geometry, including waveguides. However, guide wires, or equivalently, larger diameter stainless steel rod-like probes are rarely coated in modern guided wave radar (GWR) applications such as tank level sensing. Far more often, Sommerfeld's line can be found in commercial practice, and its propagation losses are often minimal, particularly when compared to propagation in free space over the same distance.
Guided wave radar is a form of time domain reflectometry (TDR). A pulse is propagated along an electromagnetic guide and range to an object is determined from the delay time of its echo pulse. The first GWR was disclosed in 1976 in U.S. Pat. No. 3,995,212, “Apparatus and Method for Sensing a Liquid with a Single Wire Transmission Line,” to G. Ross. Both Goubau and Ross used a horn as a transition device between a coaxial feed cable and a single wire transmission line. The horn serves two purposes: (1) conversion of electric field geometry and (2) impedance matching. A horn with a wire passing through it can be considered to be a tapered coaxial line with a smoothly changing characteristic impedance. The aperture of a horn launcher does not directly affect its gain as one might expect with a conventional free-space horn. The reason for aperture independence is the guide wire already “beams” energy from the horn—beam shaping by the horn is not required. Consequently, the primary parameter of interest in launcher horn design is impedance matching. The longer the horn and the more its mouth flares back like a tuba horn, the better the impedance matching. The horn acts as a distributed impedance matching transformer with a low impedance at its neck and a high impedance at its mouth.
While the horn is simple, it is often too large for many practical applications, e.g., tank fill level sensing, where small openings in the tank are often the only available apertures through which a GWR probe can be inserted. Another limitation to the horn is it often casts reflections from its aperture since the transition from the coaxial geometry at the aperture to the open geometry of the guide wire introduces an impedance discontinuity. U.S. Pat. No. 6,452,467, “Material Level Sensor Having a Wire-Horn Launcher,” to the present inventor discloses an open wire or leaf arrangement for the shape of a horn that provides a much smoother impedance transition to the guide wire. However, it too requires a larger physical dimension that can be tolerated in some applications.
A flat plate launcher was disclosed in U.S. Pat. No. 5,609,059, “Electronic Multi-Purpose Material Level Sensor,” 1997, to the present inventor. A flat or slightly curved launcher plate has the considerable advantage that the plate can be formed by the tank wall, so only a small tank aperture is needed through which a guide wire or rod-like probe can be inserted. This feature contributed to wide commercial success of the flat plate launcher of the '059 patent.
The plate launcher exhibits a sharp impedance discontinuity between a coaxial feed line impedance of, for example, 50 ohms and a guide wire impedance of, for example, 500 ohms. This discontinuity introduces a large reflection at the coax/launcher interface. The '059 patent discloses an apparatus that detects this reflection, termed a fiducial pulse, and uses it as a start-of-measurement reference. Measuring from the fiducial pulse has at least two key advantages: (1) time delays in the transceiver and the feed line to the launcher are not included in the range measurement, and (2) the measurement is referenced to the top of the tank (where the guide wire is often inserted) as is common in industrial practice with other sensor technologies. Unfortunately, the large reflection from the launcher plate obscures desired short-range echoes. Thus, it is difficult to sense a full condition in a tank, or to accurately measure tanks levels when nearly full. In commercial parlance, there is a “dead space” that is often specified on commercial GWR level sensors. Some commercial GWR devices digitally store the launcher reflection and subtract it from subsequent readings. This technique for removing launcher reflections in software is effective only if the launcher conditions remain the same, e.g., assuming the GWR apparatus is not mounted on a tank having different characteristics than those during digital storage of the launcher reflection.
A launcher is needed that combines the best features of both the horn and the plate with none of the drawbacks. A launcher is needed that casts no reflections so short range echoes can easily be measured. In addition, an impedance-matched low reflection launcher is needed that is compact in size for insertion through small apertures.